[Suggested files] Add suggestion index to app launch data.

We use AppLaunchData objects to send training signals about app launches
to our ranking backend. We don't set one field of this object, the
index of the launched item, which we now need for metrics. This CL adds
it in.

Bug: 1034842
Change-Id: I908c2b9072d60ae094caf51d76aa4a6be31f4dfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2099349
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Tony Yeoman <tby@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749709}

chrome/browser/ui/app_list/app_list_client_impl.cc

@@ -114,6 +114,7 @@ void AppListClientImpl::OpenSearchResult(const std::string& result_id,
       app_list::RankingItemTypeFromSearchResult(*result);
   app_launch_data.launch_type = launch_type;
   app_launch_data.launched_from = launched_from;
+  app_launch_data.suggestion_index = suggestion_index;
 
   if (launch_type == ash::AppListLaunchType::kAppSearchResult &&
       launched_from == ash::AppListLaunchedFrom::kLaunchedFromSearchBox &&